Hydrolysis-stable film comprising a polyester with a hydrolysis stabilizer and process for its production and its use

ABSTRACT

The invention relates to a hydrolysis-stable polyester film comprising a polyester, whose thickness is preferably in the range from 0.4 to 500 μm. The film comprises polyester and at least one hydrolysis stabilizer and is distinguished by its low hydrolysis rate. The invention furthermore relates to a process for the production of the film and its use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German parent application 10 2004044 326.2 which is hereby incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

The invention relates to a hydrolysis-stable film comprising apolyester, whose thickness is in the range from 0.4 to 500 μm. The filmcomprises at least one hydrolysis stabilizer and is distinguished by itslow hydrolysis rate. The invention furthermore relates to a process forthe production of the film and its use.

BACKGROUND OF THE INVENTION

Films comprising polyesters in the stated thickness range aresufficiently well known. The disadvantage of such polyester films,however, is their tendency to hydrolyze, in particular at temperaturesabove the glass transition temperature of the respective polyester.Here, tendency to hydrolyze is understood as meaning the property of thepolyester to undergo hydrolytic degradation under humid conditions,which is evident, for example, from a reduction of the IV or SV value.This is a limiting factor for the use of polyester films, particularlyin applications with a relatively high thermal load, such as in filmcapacitors, cable sheathing, ribbon cables or engine-protection films,but also in long-term applications, such as in glazing and outdoorapplications.

The tendency to hydrolyze is particularly pronounced in the case ofaliphatic polyesters, but also in the case of aromatic polyesters, suchas PBT and PET. If the tendency of PET to hydrolyze becomes too greatfor the application, it is necessary to rely on the somewhat morehydrolysis-stable PEN or even on other polymers, such as, for example,polyetherimides or polyimides. However, these are substantially moreexpensive than PET and, for economic reasons, are therefore often not asolution.

It has therefore already been proposed to improve the hydrolysisstability of polyester films by incorporating hydrolysis stabilizers.

The more hydrolysis-stable polyester raw materials which are obtained byusing carbodiimides, and fibers and films produced therefrom, are known(U.S. Pat. No. 5,885,709, EP-A-0 838 500, CH-A-621 135). However, filmswhich are produced from such raw materials tend to emit, in gaseousform, both in the production and in subsequent use, isocyanates andother byproducts and degradation products which irritate the mucousmembrane or are harmful to health. This is a much greater problem in thecase of sheet-like structures, such as a film having a large surfacethan in the case of injection-molded parts or the like.

Hydrolysis stabilizers based on epoxy groups also lead to hydrolysisstabilization and are described, for example, in EP-A-0 292 251 or U.S.Pat. No. 3,657,191. However, these compounds are based on the productionof oxirane rings by means of epichlorohydrin and, inter alia owing totheir terminal epoxy groups, tend to eliminate low molecular weighttoxic compounds on heating, so that problems similar to those with theuse of carbodiimides are associated with the use of these substances.Moreover, their incorporation into the polyester matrix is insufficient,which leads to long reaction times and, in the case of orientedpolyester films, to a considerable undesired haze.

Moreover, known hydrolysis stabilizers, such as carbodiimides and othersubstances, such as those described in EP-A-0 292 251, have thedisadvantage that in some cases they lead to considerable increases inmolecular weight (increase in viscosity) in the polymer during extrusionand thus make the extrusion process unstable and difficult to control.

SUMMARY OF ADVANTAGEOUS EMBODIMENTS OF THE INVENTION

It is an object of the present invention to provide a hydrolysis-stablepolyester raw material which avoids the described disadvantages of theprior art.

The object is achieved by a polyester film which, in addition topolyester, preferably comprises 0.1-20% by weight, based on the weightof the film, of a hydrolysis stabilizer based on epoxidized fatty acidalkyl esters and/or epoxidized fatty acid glycerides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary hazemeter which may be used tomeasure the haze of films in accordance with the invention.

DETAILED DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS OF THE INVENTION

The film comprises a polyester as the main component. Suitablepolyesters are, for example, polyethylene terephthalate (PET),polyethylene naphthalate (PEN), polybutylene terephthalate (PBT),polytrimethylene terephthalate (PTT), bibenzene-modified polyethyleneterephthalate (PETBB), bibenzene-modified polybutylene terephthalate(PBTBB), bibenzene-modified polyethylene naphthalate (PENBB) or mixturesthereof, PET, PBT, PEN and PPT and mixtures and copolyesters thereofbeing preferred.

For the preparation of the polyesters, isophthalic acid (IPA), trans-and/or cis-1,4-cyclohexanedimethanol (c-CHDM, t-CHDM or c/t-CHDM) andother suitable dicarboxylic acid components (or dicarboxylic esters) anddiol components can also be used in addition to the main monomers, suchas dimethyl terephthalate (DMT), ethylene glycol (EG), propylene glycol(PG), 1,4-butanediol, terephthalic acid (TA), benzenedicarboxylic acidand/or 2,6-naphthalenedicarboxylic acid (NDA).

Polymers in which the dicarboxylic acid component comprises 90% byweight (based on the total amount of the dicarboxylic acid component) ormore, in particular 95% by weight or more, of TA are preferred.Thermoplastics in which the diol component comprises 90% by weight ormore, in particular 93% by weight (based on the total amount of thediols) or more, of EG are furthermore preferred. Polymers in which thediethylene glycol fraction, based on the total polymer, is in the rangefrom 0.5 to 2% by weight are also preferred. The hydrolysis stabilizeris not taken into account in any of the quantity data mentioned in thisparagraph.

Other suitable polyesters are aliphatic polyesters, such aspolyhydroxybutyrate (PHB) and its copolymer with polyhydroxyvalerate(PHV), polyhydroxybutyrate-valerate (PHBV), poly(e-caprolactone) (PCL),SP 3/6, SP 4/6 (consisting of 1,3-propanediol/adipate or1,4-butanediol/adipate), polycaprolactam or generally polyesterscomprising adipic acid, and the esters of other aliphatic carboxylicacids.

The film according to the invention may furthermore comprise inorganicor organic particles which are required for establishing the surfacetopography or visual appearance. The amount of particles used isdependent on the substances used and their particle size. The latter isin general in the range from 0.01 to 30.0, preferably from 0.1 to 5.0and in particular from 0.3 to 3.0 μm.

Suitable compounds for achieving the roughness are, for example, calciumcarbonate, apatite, silica, titanium dioxide, alumina, crosslinkedpolystyrene, crosslinked polymethylmethacrylate (PMMA), zeolites andother silicates, such as aluminum silicates. These compounds aregenerally used in amounts of from 0.05 to 5% by weight, preferably from0.1 to 0.6% by weight (based on the weight of the film).

In addition to the additives mentioned, the film may additionallycomprise further components, such as flameproofing agents and/orfree-radical scavengers and/or other polymers, such as polyetherimides.

The film according to the invention comprises a hydrolysis stabilizer,which is preferably metered in by means of the masterbatch technologydirectly during the film production, the proportion of the hydrolysisstabilizer preferably being in the range from 0.1 to 20.0% by weight,preferably from 1.0 to 6.0% by weight and particularly preferably 1.5 to4.5% by weight, based on the weight of the crystalline thermoplastic. Inthe masterbatch, the proportion of the hydrolysis stabilizer is ingeneral from 2.0 to 50.0% by weight, preferably from 4.0 to 20.0% byweight, based in each case on the total weight of the masterbatch.

Suitable hydrolysis stabilizers are epoxidized fatty acid alkyl estersand/or epoxidized fatty acid glycerides.

Preference is given here to mixtures which are composed of

-   a) from 90 to 10% by weight (based on the mixture) of long-chain    epoxidized fatty acid alkyl esters and-   b) from 10to 90% by weight (based on the mixture) of epoxidized    fatty acid glycerides.

Particularly suitable hydrolysis stabilizers are those which have anepoxy oxygen content of at least 0.5% by weight, preferably at least 1.5and more preferably more than 2.0% by weight.

Suitable epoxidized fatty acid glycerides are, for example, epoxidizedsoybean oil, epoxidized linseed oil, epoxidized rapeseed oil, epoxidizedsunflower oil and epoxidized fish oil (the composition of the statedoils, in particular the type and amount of the fatty acids present, isdescribed for example in Römpp Chemie Lexikon, 10th edition, GeorgThieme Verlag, Stuttgart). Epoxidized fatty acid alkyl esters used arepreferably the thermally stable 2-ethylhexyl esters of unsaturated fattyacids or fatty acid mixtures of the fatty acids on which rapeseed oil,linseed oil, soybean oil, or fish oil are based, which preferentiallyhave epoxy contents of 1.5-15% by weight of epoxy oxygen (based on theepoxidized fatty acid alkyl esters), preferably 4-8% by weight.

To prepare the mixtures the epoxidized glycerides are mixed with theepoxidized fatty acid esters in the amounts indicated above attemperatures from 20 to 120° C., preferably at 30 to 60° C., givingliquid products.

The hydrolysis stabilizer is added to the thermoplastic of the filmpreferably by means of the masterbatch technology. For this purpose, itis first dispersed in a carrier material. The thermoplastic itself, forexample polyethylene terephthalate or other polymers which arecompatible with the thermoplastic, are suitable as carrier material.After the masterbatch has been metered into the thermoplastic for thefilm production, the components of the masterbatch melt during theextrusion and are thus dissolved or finely dispersed in thethermoplastic.

The preparation of the masterbatch is expediently effected by one of thetwo following processes:

-   1. The carrier material and the liquid hydrolysis stabilizer are    melted in an extruder, preferably a twin-screw extruder, mixed and    then extruded through a die, quenched and granulated. A process in    which the carrier material (polymer) is first melted in the extruder    and, after devolatilization, the liquid hydrolysis stabilizer is    metered directly into the melt is preferred.-   2. Addition of the hydrolysis stabilizer during preparation of the    carrier material. The hydrolysis stabilizer is pumped directly into    the line through which the prepared carrier material (polymer) is    pumped to the dies prior to granulation. However, this method has    the disadvantage that the mixing of hydrolysis stabilizer and    carrier material is not as good as in method 1. Owing to the    generally very high viscosity differences, metering into the    reaction reactor in the preparation of the carrier material    (polymer) generally leads to even poorer mixing.

If the film according to the invention comprises particles, it hasproven to be advantageous if these particles are also already present inthe hydrolysis stabilizer masterbatch, since particles positivelyinfluence the distribution of the hydrolysis stabilizer. It has provento be particularly advantageous for the distribution if the carriermaterial comprising the hydrolysis stabilizer comprises at least 0.3,preferably more than 0.75, % by weight (based on the masterbatch) ofSiO₂ and/or Al₂O₃ particles.

Furthermore, it has proven to be advantageous if a stabilizer in theform of a free-radical scavenger is added to the masterbatch (or to thepolymer raw material) which comprises the hydrolysis stabilizer, sincethis counteracts the loss of active oxirane groups in the extrusion dueto free radical secondary reactions. The film according to the inventionexpediently comprises such stabilizers as free-radical scavengers orheat stabilizers in amounts of from 50 to 15 000 ppm, preferably from100 to 5000 ppm, particularly preferably from 300 to 1000 ppm, based onthe weight of the film. The stabilizers added to the polyester rawmaterials are selected as desired from the group consisting of theprimary stabilizers, such as sterically hindered phenols or secondaryaromatic amines, or from the group consisting of the secondarystabilizers, such as thioethers, phosphites and phosphonites, and zincdibutyldithiocarbamate or synergistic mixtures of primary and secondarystabilizers. The phenolic stabilizers are preferred. The phenolicstabilizers include in particular sterically hindered phenols,thiobisphenols, alkylidenebisphenols, alkylphenols, hydroxybenzylcompounds, acylaminophenols and hydroxyphenylpropionates (correspondingcompounds are described, for example, in “Kunststoffadditive” [PlasticsAdditives], 2nd Edition, Gächter Müller, Carl Hanser-Verlag, and in“Plastics Additives Handbook”, 5th Edition, Dr. Hans Zweifel, CarlHanser-Verlag). The stabilizers having the following CAS numbers areparticularly preferred: 6683-19-8, 36443-68-2, 35074-77-2, 65140-91-2,23128-74-7, 41484-35-9, 2082-79-3 and IRGANOX® 1222 from CibaSpecialities, Basel, Switzerland, in particular embodiments the typesIRGANOX® 1010, IRGANOX® 1222, IRGANOX® 1330 and IRGANOX® 1425 ormixtures thereof being preferred.

Apart from the addition of the hydrolysis stabilizer by means of themasterbatch technology, the hydrolysis stabilizer can also be addeddirectly during film production. However, this leads to good resultsonly if twin-screw extruders are used. Here too, the best results withregard to film quality and hydrolysis effect are obtained if thehydrolysis stabilizer is metered directly into the melt in therespective extruder after the devolatilization zone.

The film according to the invention is generally produced by extrusionprocesses known per se and has one or more layers, the hydrolysisstabilizer preferably being present in all layers, and embodiments inwhich not all layers are provided with the hydrolysis stabilizer alsobeing possible.

In the process for producing the films according to the invention, theprocedure is expediently adopted in which the corresponding melts areextruded through a flat-film die, the resultant film is drawn off andquenched in the form of a substantially amorphous prefilm on one or moreroll(s) (chill roll) for solidification, the film is then reheated andbiaxially stretched (oriented), and the biaxially stretched film isheat-set. In the region of the extrusion, it has proven to beadvantageous if temperatures of 295° C. are not exceeded. It isparticularly advantageous if the region of the die and especially theregion of the die lip and the immediate vicinity is not warmer than 290°C., preferably not warmer than 285° C. and particularly preferably notwarmer than 275° C.

The biaxial stretching is generally carried out sequentially.Preferably, stretching is effected first in the longitudinal direction(i.e. in the machine direction=MD) and then in the transverse direction(i.e. perpendicular to the machine direction=TD). This leads to anorientation of the molecular chains. The stretching in the longitudinaldirection can be carried out with the aid of two rolls running atdifferent speeds according to the intended stretching ratio. For thetransverse stretching, a corresponding tenter frame is generally used.

The temperature at which the stretching is carried out can vary within arelatively large range and depends on the desired properties of thefilm. In general, the longitudinal and also the transverse stretching iscarried at T_(g)+10° C. to T_(g)+60° C. (T_(g)=glass transitiontemperature of the film). The longitudinal stretching ratio is ingeneral in the range from 2.0:1 to 6.0:1, preferably from 3.0:1 to4.5:1. The transverse stretching ratio is in general in the range from2.0:1 to 5.0:1, preferably from 3.0:1 to 4.5:1, and that of theoptionally performed second longitudinal and transverse stretching isfrom 1.1:1 to 5.0:1.

The first longitudinal stretching can optionally be carried outsimultaneously with the transverse stretching (simultaneous stretching).It has proven to be particularly advantageous if the stretching ratio inthe longitudinal and transverse directions is in each case greater than3.0.

In the subsequent heat-setting, the film is kept at a temperature offrom 150° C. to 260° C., preferably from 200 to 245° C., for about 0.1to 10 s. After the heat-setting or beginning therein, the film isrelaxed by from 0 to 15%, preferably by from 1.5 to 8%, in thetransverse and optionally also in the longitudinal direction, and thefilm is cooled and wound in a conventional manner.

A film produced in this manner has substantially less tendency tohydrolyze both at room temperature and at temperatures up to 210° C.than an unmodified polyester film. The stabilization is substantiallyindependent of the film thickness and the temperature in a measuringrange of 25 to 210° C. Thus, for example, a 12 μm thick single-layer PETfilm (DEG content 1% by weight and initial SV value of 720) comprising2% by weight of a hydrolysis stabilizer mixture comprising one partepoxidized soybean oil having an epoxide oxygen content of 6.3% byweight and one part epoxidized stearic acid 2-ethylhexyl ester having anepoxide oxygen content of 4.5% by weight (epoxide oxygen content of themixture 5.3% by weight) still has an SV value of above 500 after 96 h inan autoclave with water vapor saturation and at 110° C. and is thereforestill mechanically stable, whereas an unstabilized film has alreadydecreased below SV 400 after this time and therefore has virtually noflexural strength. The stabilized film withstands said conditions for50% longer before it reaches the critical limit of 400 SV units. Thesame relative time gain is also found at 80° C. and at 170° C.

It was particularly surprising that, in spite of the good long-termstabilization to hydrolysis, no undesired increase in viscosity in theextruder was found and no increased gel or speck level was observed inthe film production.

Films which are stabilized by means of said hydrolysis stabilizers areoutstandingly suitable for the production of products which comprisepolyester films and are designed either for a long life (more than 1year) or which are subjected to relatively high temperatures (greaterthan 80° C.), in particular at high humidity, during their use, or foroutdoor applications.

Thus, they are outstandingly suitable for the production of filmcapacitors (preferred thickness range 0.4-12 μm). These can be producedby the known conventional methods and process sequences (inter alia,metallization, assembly, winding, Schoop process, contacting, potting,etc.) and have a substantially longer life compared with theconventional polyester film capacitors and, in contrast to capacitorsalready described and comprising carbodiimide stabilizers, do not leadto the emission of isocyanates harmful to health, even on strongheating. For the production of capacitors, it has proven to beadvantageous if the film have a longitudinal shrinkage of less than 4%and a transverse shrinkage of less than 1% at 200° C., since they arethen particularly suitable for the production of SMD capacitors. Thefilms according to the invention have such low shrinkage values.

For example, ribbon cables in automobiles constitute a furtherapplication. For this purpose, films (preferably 12-200 μm) arelaminated with copper by means of a heat-seal adhesive (e.g. EKPheat-seal lacquer 230 from EKP Verpackungslacke GmbH (Germany)).Composites which comprise polyester with hydrolysis stabilizer withstandthe mechanical loads (including vibrations) occurring in automobiles formuch longer than composites comprising conventional polyester films.However, it should be ensured here that the adhesives, too, aresubstantially insensitive to hydrolysis (in the case of polyester-basedadhesives, treatment with said hydrolysis stabilizers is advisable).

In the following embodiments, the measurement of the individualproperties is effected according to the stated standards or methods.

Methods of Measurement

Standard Viscosity (SV)

The standard viscosity SV is measured—based on DIN 53726—by themeasurement of the relative viscosity η_(rel.) of a 1% strength byweight solution in dichloroacetic acid (DCA) in an Ubbelohde viscometerat 25° C. The SV value is defined as follows:SV=(η_(rel.)−1)·1000Roughness

The roughness Ra of the film is determined according to DIN 4768 at acut-off of 0.25 mm.

Shrinkage

The thermal shrinkage was determined using square film samples having anedge length of 10 cm. The samples are cut out so that one edge isparallel to the machine direction and one edge is perpendicular to themachine direction. The samples are measured exactly (the edge length L₀is determined for each machine direction TD and MD, L_(0 TD) andL_(0 MD)) and are heated for 15 min at the stated shrinkage temperature(200° C. here) in a forced-circulation drying oven. The samples areremoved and are exactly measured at room temperature (edge length L_(TD)and L_(MD)) The shrinkage is obtained from the equationShrinkage [%]MD=100·(L _(0 MD) −L _(MD))/L _(0 MD)Shrinkage [%]TD=100·(L _(0 TD) −L _(TD))/L _(0 TD)Measurement of the Haze

The measurement is effected using the Hazegard hazemeter XL-211 from BYKGardner. The measuring apparatus is switched on 30 min before themeasurement. It should be ensured that the light beam passes through thesphere concentrically with the outlet aperture.

Production, Shape and Number of Samples

5 samples each, having a size of 100 by 100 mm, are cut out from thefilm to be investigated. The longitudinal direction and transversedirection are marked on the edge since the measurements are effected inboth machine directions.

Measurement of the Haze

-   -   Press switch 1 “OPEN”    -   Set switch 2 to “X10” and calibrate digital display to 0.00        using the “Zero” button    -   Switch over switch 1 to “Reference” and switch 2 to “X1”    -   Using the “Calibrate” button, bring the digital display to 100    -   Place sample in longitudinal direction    -   Read display value for the transparency    -   Calibrate the digital display to 100 using the “Calibrate”        button    -   Set switch 1 to “OPEN”    -   Read display value for the haze in the longitudinal direction    -   Turn sample into transverse direction    -   Read display value for the haze in the transverse direction        Evaluation

The haze is obtained by averaging the respective 5 individual values(longitudinal and transverse).

Autoclaving

The films (10·2 cm) are suspended from a wire in the autoclave (AdolfWolf SANOklav type ST-MCS-204), and the autoclave is filled with 2 l ofwater. After being closed, the autoclave is heated. At 100° C., the airis displaced by the steam via the discharge valve. This is closed afterabout 5 min, after which the temperature increases to 110° C. and thepressure to 1.5 bar. After 24 h, the autoclave is automatically switchedoff, and the films are removed after the discharge valve has beenopened. The SV value of said films is then determined.

EXAMPLES

Hydrolysis Stabilizer 1

A steel reactor equipped with an internal thermometer was charged atroom temperature with 10 kg of epoxidized soybean oil having an epoxyoxygen content of 6.3% by weight, to which 10 kg of epoxidized stearicacid 2-ethylhexyl ester having an epoxy oxygen content of 4.5% by weightwere added, after which the mixture was stirred at 25° C. over thecourse of 30 minutes to give a clear, homogeneous solution. The epoxyoxygen content thereafter is 5.3% by weight.

Hydrolysis Stabilizer 2

A steel reactor equipped with an internal thermometer was charged atroom temperature with 15 kg of epoxidized linseed oil having an epoxyoxygen content of 9.1% by weight, to which 5 kg of epoxidized stearicacid 2-ethylhexyl ester having an epoxy oxygen content of 4.5% by weightwere added, after which the mixture was stirred at 45° C. over thecourse of 30 minutes to give a clear, homogeneous solution. The epoxyoxygen content of the mixture thereafter is 7.2% by weight.

Hydrolysis Stabilizer 3

Epoxidized linseed oil having an epoxy oxygen content of 9.1% by weight,with 2% by weight of Irganox 1010.

Preparation of Raw Materials (Polymers Stabilized to Hydrolysis)

PET Comprising Hydrolysis Stabilizer 1

A polyethylene terephthalate raw material comprising 10 000 ppm ofSylobloc 44H (Grace) and 5000 ppm of Aerosil TT600 (Degussa) and 3000ppm of Irganox 1010 (Ciba) and a DEG content of 1% is melted in atwin-screw extruder from Coperion and mixed with 8% by weight ofhydrolysis stabilizer 1. Hydrolysis stabilizer 1 was metered by means ofa pump directly into the melt after the devolatilization zone. SV value790.

PET Comprising Hydrolysis Stabilizer 2

Polyethylene terephthalate raw material RT49 (R1) from Invista wasmelted in a twin-screw extruder from Coperion and mixed with 4% byweight of hydrolysis stabilizer 2. Hydrolysis stabilizer 2 was meteredby means of a pump directly into the melt after the devolatilizationzone. SV value 790.

PET Comprising Hydrolysis Stabilizer 3

Polyethylene terephthalate raw material RT49 (R1) from Invista wasmelted in a twin-screw extruder from Coperion and mixed with 4% byweight of hydrolysis stabilizer 3. Hydrolysis stabilizer 3 was meteredby means of a pump directly into the melt after the devolatilizationzone. SV value 790.

Further Raw Materials Used

-   Raw material R1 PET (type RT49, Invista Germany), SV value 790-   Masterbatch MB1 1.0% by weight of Sylobloc 44H, 0.50% of Aerosil    TT600 and 98.5% by weight of PET; SV value 790; DEG content of 1% by    weight    Film Production (Capacitor Film) Process 1:

Thermoplastic chips were mixed according to the ratios stated in theexamples and were precrystallized in a fluidized-bed drier at 155° C.for 1 min, then dried for 3 h in a shaft drier at 150° C. and extrudedat 278° C. The molten polymer was taken off from a die via a take-offroll. The thickness of this prefilm was 29 μm. The film was stretched bya factor of 3.8 in the machine direction at 116° C., and transversestretching by a factor of 3.7 was carried out in a frame at 110° C.Thereafter, the film was heat-set at 230° C. and relaxed in thetransverse direction by 8% at temperatures of 200-180° C. The final filmthickness was 2 μm.

Film Production for Ribbon Cable Process 2:

Thermoplastic chips were mixed according to the ratios stated in theexamples and extruded at 278° C. in a twin-screw extruder(JapanSteelWorks). The molten polymer was taken off from a die via atake-off roll. The thickness of this prefilm was 530 μm. The film wasstretched by a factor of 3.4 in the machine direction at 116° C., andtransverse stretching by a factor of 3.1 was carried out in a frame at110° C. Thereafter, the film was heat-set at 225° C. and relaxed in thetransverse direction by 3% at temperatures of 200 to 180° C. The finalfilm thickness was 50 μm.

The properties of the films produced are shown in the table below.

TABLE Comparative Comparative Example 1 Example 2 Example 3 Example 4example 1 example 2 PET with hydrolysis in % by 25 25 stabilizer 1weight PET with hydrolysis in % by 90 stabilizer 2 weight PET withhydrolysis in % by 90 stabilizer 3 weight R1 in % by 75 75 75 90 weightMB1 in % by 10 10 25 10 weight Process No. 1 2 2 2 1 2 Film thickness inμm 2 50 50 50 2 50 Ra value in nm 85 90 125 93 82 97 SV value 765 785788 784 763 787 SV value after 96 h 505 530 511 500 391 399 at 110° C.in an autoclave with water saturation

1. A biaxially oriented polyester film comprising polyester and ahydrolysis stabilizer comprising an effective amount of a mixture ofepoxidized fatty acid alkyl esters and epoxidized fatty acid glycerides,said film retaining a higher solution viscosity value after 96 hours at110° C. in an autoclave with water saturation than a comparable filmformed with said effective amount of said epoxidized fatty acidglyceride alone.
 2. The polyester film as claimed in claim 1, whereinsaid film is a single- or multilayer film.
 3. The polyester film asclaimed in claim 2, wherein said film is a multilayer film.
 4. Thepolyester film as claimed in claim 3, wherein the hydrolysis stabilizeris present in all of the layers of the multilayer film.
 5. The polyesterfilm as claimed in claim 1, which comprises organic or inorganicparticles.
 6. The polyester film as claimed in claim 1, which comprisesflame retardant and/or free-radical scavenger.
 7. The polyester film asclaimed in claim 1, wherein from 0.1 to 20.0% by weight, based on theweight of the film, of hydrolysis stabilizer is present.
 8. Thepolyester film as claimed in claim 1, wherein the hydrolysis stabilizeris a mixture comprising (a) from 90 to 10% by weight, based on themixture, of epoxidized fatty acid alkyl esters and (b) from 10 to 90% byweight, based on the mixture, of epoxidized fatty acid glycerides. 9.The polyester film as claimed in claim 1, wherein the hydrolysisstabilizer has an epoxy oxygen content of at least 0.5% by weight. 10.The polyester film as claimed in claim 1, wherein the epoxidized fattyacid glycerides have been selected from one or more of the followingsubstances: epoxidized soybean oil, epoxidized linseed oil, epoxidizedrapeseed oil, epoxidized sunflower oil, and epoxidized fish oil.
 11. Thepolyester film as claimed in claim 1, wherein the epoxidized fatty acidalkyl esters have been selected from one or more of the followingsubstances: thermally stable 2-ethylhexyl esters of the unsaturatedfatty acids or fatty acid mixtures of the fatty acids on which rapeseedoil, linseed oil, soybean oil, or fish oil are based.
 12. The polyesterfilm as claimed in claim 1, wherein the epoxidized fatty acid alkylesters have epoxy oxygen contents of from 1.5 to 15% by weight of epoxyoxygen.
 13. A process for production of a polyester film as claimed inclaim 1, wherein a melt comprising (i) polyester and (ii) a masterbatchcomprising polyester and a hydrolysis stabilizer based on epoxidizedfatty acid alkyl esters and/or on epoxidized fatty acid glycerides isextruded through a flat-film die, the resultant film is drawn off andquenched in the form of a substantially amorphous prefilm on one or morerolls for solidification, the film is then reheated and biaxiallystretched, and the biaxially stretched film is heat-set.
 14. Filmcapacitors, cable sheathing, ribbon cables, or engine-protection filmscomprising a film according to claim
 1. 15. Glazing or outdoorapplication film comprising a film according to claim 1.